The conventional method of metering liquid consumption is to place a so-called paddle wheel or other movable device in the liquid flow. The accuracy of such standard flow-meters is, however, dependent on a number of factors including temperature, pressure, the specific gravity and viscosity of the liquid, and the presence of any air in the liquid. Such flow-meters may also vary in their characteristics as a function of time, hence requiring periodic calibration.
This problem is especially noticeable when pumping ink to a printing press, because the ink is typically closer in viscosity to tooth paste than to water, is thixotropic (i.e. the viscosity varies with conditions), and is pumped in frequent stop and start cycles.
In an endeavor to overcome the difficulties involved in accurately monitoring the supply of such a relatively viscous liquid, a flow-meter has been designed that employs a meshed oval gear structure to replace the paddle wheel. However, this meter has suffered from essentially the same problems as the paddle wheel meters. Moreover, it has been particularly prone to jamming and has experienced premature wear due to the presence of foreign particles in the ink.
Typically, commercial operations maintain a supply of liquid, e.g. ink, in a reservoir and feed a supply of such liquid to one or more usage stations (each station could, for example, be a printing press) by means of a conventional, positive displacement pump that makes the liquid available under pressure at each station. Such a pump will normally be powered by air, usually received at about 60 p.s.i. or higher, the air being exhausted to atmosphere after each stroke. In the liquid line the pump takes in liquid from the reservoir under gravity feed, i.e. at approximately atmospheric pressure, and delivers a predetermined volume of the liquid with every stroke at a relatively high pressure, e.g. 1500 p.s.i.
Each pump is set to maintain a constant liquid pressure in its supply line for a given air pressure setting. When a usage station takes an amount of the liquid the line pressure drops. The pump is sensitive to this pressure drop and it automatically starts up to re-establish the desired supply line pressure.